As electrical storage devices, for example, secondary batteries such as lithium ion batteries, nickel hydrogen batteries, or lead storage batteries, or electric double layer capacitors are known. The further downsizing of electrical storage devices is sought because of the downsizing of mobile devices, limitations of installation spaces, etc., and lithium ion batteries in which the energy density is high have been receiving attention. Such a lithium ion battery uses a configuration which covers the entirety of the battery with a packaging material including an aluminum foil layer to prevent moisture from entering through the surface, and is referred to as an aluminum laminate lithium ion battery. The aluminum laminate lithium ion battery, for example, forms a recess on a part of the packaging material by cold forming, accommodates the battery elements (a positive electrode, a separator, a negative electrode, an electrolytic solution, etc.) in the recess, and folds the remaining portions of the packaging material and seals the edge portions by heat-sealing, and an embossed lithium ion battery (hereinafter, referred to as “single-sided formed battery”) is formed thereby.
The energy density of the lithium ion battery increases as the depth of the recess formed by cold forming increases. However, pinholes or breaking readily occurs during forming in the packaging material as the formed recess becomes deeper, leading to more deterioration in formability. Therefore, a stretched polyamide film or the like has been used for the base layer of the packaging material to protect a metal foil such as aluminum foil. As an example, PTL 1 proposes use of a film for the base layer, wherein the film's tensile strengths in four directions, 0°, 45°, 90° and 135°, until the occurrence of breakage in a tensile test are 150 N/mm2, and the elongations in the four directions are 80% or more.